fix key directory hash calculation code

Fix Key directory hash calculation code to behave as it did before version
3.20120227 when a key contains non-ascii.

The hash directories for a given Key are based on its md5sum.
Prior to ghc 7.4, Keys contained raw, undecoded bytes, so the md5sum was
taken of each byte in turn. With the ghc 7.4 filename encoding change,
keys contains decoded unicode characters (possibly with surrigates for
undecodable bytes). This changes the result of the md5sum, since the md5sum
used is pure haskell and supports unicode. And that won't do, as git-annex
will start looking in a different hash directory for the content of a key.

The surrigates are particularly bad, since that's essentially a ghc
implementation detail, so could change again at any time. Also, changing
the locale changes how the bytes are decoded, which can also change
the md5sum.

Symptoms would include things like:

* git annex fsck would complain that no copies existed of a file,
  despite its symlink pointing to the content that was locally present
* git annex fix would change the symlink to use the wrong hash
  directory.

Only WORM backend is likely to have been affected, since only it tends
to include much filename data (SHA1E could in theory also be affected).

I have not tried to support the hash directories used by git-annex versions
3.20120227 to 3.20120308, so things added with those versions with WORM
will require manual fixups. Sorry for the inconvenience!

2 files changed, 17 insertions, 13 deletions

diff --git a/Utility/FileSystemEncoding.hs b/Utility/FileSystemEncoding.hs
index 048323ee3..fe4202a75 100644
--- a/Utility/FileSystemEncoding.hs
+++ b/Utility/FileSystemEncoding.hs
@@ -7,31 +7,37 @@
 
 module Utility.FileSystemEncoding where
 
-import System.IO
+import qualified GHC.Foreign as GHC
+import qualified GHC.IO.Encoding as Encoding
 import Foreign.C
-import GHC.Foreign as GHC
-import GHC.IO.Encoding
+import System.IO
+import System.IO.Unsafe
 
 {- Sets a Handle to use the filesystem encoding. This causes data
  - written or read from it to be encoded/decoded the same
  - as ghc 7.4 does to filenames etc. This special encoding
  - allows "arbitrary undecodable bytes to be round-tripped through it". -}
 fileEncoding :: Handle -> IO ()
-fileEncoding h = hSetEncoding h =<< getFileSystemEncoding
+fileEncoding h = hSetEncoding h =<< Encoding.getFileSystemEncoding
 
 {- Marshal a Haskell FilePath into a NUL terminated C string using temporary
  - storage. The FilePath is encoded using the filesystem encoding,
  - reversing the decoding that should have been done when the FilePath
  - was obtained. -}
 withFilePath :: FilePath -> (CString -> IO a) -> IO a
-withFilePath fp f = getFileSystemEncoding >>= \enc -> GHC.withCString enc fp f
+withFilePath fp f = Encoding.getFileSystemEncoding
+	>>= \enc -> GHC.withCString enc fp f
 
 {- Encodes a FilePath into a String of encoded bytes, applying the
  - filesystem encoding.
  -
- - This does not do any IO, beyond allocating a C buffer. GHC does not
- - seem to provide a pure way to do this conversion. -}
-encodeFilePath :: FilePath -> IO String
-encodeFilePath fp = do
-	enc <- getFileSystemEncoding
-	GHC.withCString enc fp $ GHC.peekCString enc
+ - This use of unsafePerformIO is belived to be safe; GHC's interface
+ - only allows doing this conversion with CStrings, and the CString buffer
+ - is allocated, used, and deallocated within the call, with no side
+ - effects.
+ -}
+{-# NOINLINE encodeFilePath #-}
+encodeFilePath :: FilePath -> String
+encodeFilePath fp = unsafePerformIO $ do
+	enc <- Encoding.getFileSystemEncoding
+	GHC.withCString enc fp $ GHC.peekCString Encoding.char8
diff --git a/Utility/StatFS.hsc b/Utility/StatFS.hsc
index 58d0b3e02..ed4c9f1cb 100644
--- a/Utility/StatFS.hsc
+++ b/Utility/StatFS.hsc
@@ -52,8 +52,6 @@ import Utility.FileSystemEncoding
 import Foreign
 import Foreign.C.Types
 import Foreign.C.String
-import GHC.IO.Encoding (getFileSystemEncoding)
-import GHC.Foreign as GHC
 
 #if defined (__FreeBSD__) || defined (__FreeBSD_kernel__) || defined (__APPLE__)
 # include <sys/param.h>